Ornithine Aminotransferase Deficiency in Gyrate Atrophy: We have isolated a gene probe for the human ornithine aminotransferase (OAT), a mitochondrial enzyme which is deficient in gyrate atrophy patients. The gene probe is a Lambda-gt11 cDNA clone which was obtained from our retinoblastoma library through a Western screening method using the anti-human OAT antibodies. The complete DNA sequence of the cDNA was obtained. The OAT cDNA is 2073 basepairs (bp) long, and appears to be a nearly full length copy of the human OAT mRNA which is approximately 2.4 Kb in a Northern analysis. Numerous tryptic peptides were obained from the pure OAT protein, and amino acid sequences of seven selected peptides were obtained by a microsequencing technique. A comparison of the amino acid sequences of the tryptic peptides to that derived from the OAT cDNA sequence revealed 111 out of 115 residues to be identical including a match of 20 consecutive amino acid residues. With our cDNA clone confirmed as a human OAT probe, we have begun to use it to examine the organization of the OAT gene in normal and gyrate atrophy patients. A gene clone for the human OAT has also been isolated and is being analyzed. Hereditary Retinoblastoma: We are investigating the molecular basis of malignant transformation in hereditary retinoblastoma using cell culture and molecular genetic techniques. In view of the published data indicating that induction of hereditary retinoblastma may involve a loss or inactivation of a gene on chromosome 13, we are investigating the possibility that the genomic DNA from normal human retina, when transfected on retinoblastoma cells, may be able to change the phenotype of retinoblastoma to that of a more 'normal' cell. To determine if retinoblastoma has a dominant or recessive malignant phenotype, reinoblastoma cells are being fused with normal cells, and the growth characteristics of the hybrid cells are being studied. Complementary DNA libraries made from the mRNAs of normal human retina and retinoblstoma are being screened for clones that represent differential expression in the two tissues, and such clones when isolated, will be identified and studied.